The selection of physical sites for installing one or more radio stations, for example base radio stations, and more specifically the optimum selection of a subset of sites from a wider set of candidate sites is a significantly relevant aspect in the field of planning, designing and developing networks, in particular third-generation cellular networks (UMTS), both for network operators that can already be found on the market, and for those operators that enter the sector for the first time.
Problems of such nature, in fact, deal both with pre-existing operators owning a network, for example of the GSM type, of which the sites available as regards installation of new radio stations, for example third-generation ones, will be partly or wholly re-used, and with new operators that will have to comply with the need of establishing how many cells have to be placed in the territory and where they have to be physically located. Obviously, even though the present invention preferably refers to the cellular network environment, it goes without saying that the invention can be used also in the wider context of telecommunications networks, particularly, for example, in the fixed networks context that provides for a designing and/or planning that requires a site selection for radio apparatuses.
As known, in the initial step of installing a complex system such as, for example, a third-generation mobile network and during the normal evolution (enlargement) of the network itself, aim of the operators is serving a specific geographic area complying with the following requirements, that in some cases are mutually in contrast:                territory electromagnetic coverage;        carrying the traffic volumes offered by the different active network services;        limiting the costs due to apparatuses installation and management.        
It is therefore important to efficiently determine the set of base radio stations to be used in order to guarantee required performances and obtain a good network development hypothesis in time (the so-called “roll-out”) upon increasing the users and deploying services that are more and more numerous, different and complex.
The prior art where the problem of optimally selecting base radio stations sites has been dealt with comprises United States patent U.S. Pat. No. 6,094,580 assigned to Nortel Networks Corporation, entitled “Method for Optimising Cell-Site Placement”, dated 25 Jul. 2000, and the Graduation Thesis prepared in Politecnico of Turin, entitled “Metodo automatico per la pianificazione di una rete UMTS mediante la valutazione di un insieme di siti candidati”, Paragraphs 2 and 3 in Chapter 6 and Chapter 7, Emanuele Silio, May 2001.
The method for selecting sites object of patent U.S. Pat. No. 6,094,580 is based on a process structured into two elementary steps in which the following are respectively used:                Linear Programming Operating Research techniques for defining an initial solution; and        an algorithm for refining the solution obtained in the previous step, of a non-recurring type.        
The used sites selection criteria depend on considerations based on the degree of obtained coverage or service, and on investment costs.
The cost function used for selecting is associated with a problem of minimum and is composed of two different terms, related to economic cost of activated cells/sites and to the cost, expressed through suitable penalties, of traffic (or alternatively geographic area) not served by the resulting network.
After having defined an initial solution obtained through Linear Programming techniques, a “greedy” algorithm operates in order to activate a subset of cells selected in the previous step. Such algorithm iteratively operates by adding a cell/site to every performed elementary step, till a specific stop condition is verified.
The proposed stop conditions are:
1) the selected cell/site does not satisfy a usefulness criterion expressed in terms of cost of incremental traffic that can be carried in case of activation; such cost is expressed by the relationship:
      r    ⁡          (      s      )        =            W      ⁡              (        s        )                    C      S      where:
W(S) is the traffic (in erlang) that can be carried by the cell/site S once activated; and
Cs is the cell/site activation cost.
The criterion to be satisfied as regards a new activation is related to the fact that the inactive cell/site, characterised by maximum usefulness r(S), satisfies the relationship:
      r    ⁡          (      s      )        =                    W        ⁡                  (          s          )                            C        S              ≥                  Min        erlang                    C        S            where the right-hand term in the inequality expresses the minimum increase of carried erlangs normalised with respect to the cell activation cost, that must be satisfied in order to activate the examined cell.
2) the selected cell/site does not satisfy a usefulness criterion expressed in terms of cost of incremental area that can be covered in case of activation; such cost is expressed by the relationship:
      r    ⁡          (      s      )        =            A      ⁡              (        s        )                    C      S      where:
A(s) is the area in square meters that can be covered by the cell/site S once activated; and
Cs is the cell/site activation cost.
The criterion to be satisfied for a new activation is related to the fact that the inactive cell/site characterised by maximum usefulness r(S) satisfies the relationship:
      r    ⁡          (      s      )        =                    A        ⁡                  (          s          )                            C        S              ≥                  Min        area                    C        S            where the right-hand term in the inequality expresses the minimum increase of covered area, normalised with respect to the cell activation cost, that must be satisfied in order to activate the examined cell.
3) the area globally covered by the system satisfies, after activating the new cell/site, the minimum coverage requirement.
The above three criteria are used as a mutual alternative. Therefore, the site selecting algorithm uses a single stop criterion and the final solution exclusively satisfies the used criterion.
The inconveniences of the proposed solution can be summarised as follows:                multiple admissibility criteria for the solution are not simultaneously used;        the criterion in item 1) does not guarantee the compliance with a minimum requirement (or constraint) related to total traffic carried by the set of activated cells (different from 100%);        the criterion in item 2) does not guarantee the compliance with a minimum requirement (or constraint) related to the area covered by the set of activated cells (different from 100%).        
It is important to note how the first mentioned inconvenience is particularly relevant, since it is not adequate to use, for example, the globally carried traffic only in order to judge a good solution, without taking into account the total area covered by the considered set of cells. In fact, leaving significantly extended areas, characterised by a scarce offered traffic, uncovered, can be a penalty from the point of view of quality and geographic continuity of services offered to radio-mobile users.
Similar considerations can obviously be made in the completely opposite case in which, as judgement parameter, only the covered area is used, without any type of evaluations related to the traffic actually carried by the system.
The Graduation Thesis “Metodo automatico per la pianificazione di una rete UMTS mediante la valutazione di un insieme di siti candidati” describes a site selecting algorithm based on an approach derived from Operating Search techniques that allows choosing, in order to activate the base radio stations, a subset of sites starting from a wider set of candidates, in which the adopted selection criteria depend on predetermined radio design parameters for third-generation networks.
In more detail, the algorithm operates in order to optimise the resulting coverage depending on parameters such as globally served area, carried traffic and distribution of cell loads ηcell and soft hand-over loads ηSHO of activated cells.
The relevant algorithm is based on guide lines of a search methodology called “Taboo Search” and uses a cost function of the multidimensional type for evaluating a solution belonging to the space of solutions (the space of solutions corresponds to the complete set of possible solutions), and elementary cells/sites activation and deactivation moves (actions) that allow exploring the space of solutions to the site selection problem, transforming a solution S1 into a different solution S2. Such algorithm uses, for evaluating the efficiency of a generic solution, a cost function composed of four different terms:FC=W1·ANS%+W2·TNS%+W3·SC+W4·SSH where:                W1, W2, W3, W4 represent the weights associated with each function term; and        the first two cost items A%NS and T%NS, respectively related to the remaining uncovered area and to the remaining traffic not carried by the set of selected cells, assume the following form:        
            A      NS      %        =                            A          TOT                -                              ∑                          i              =              1                                      N              activecells                                ⁢                      A            i            service                                      A        TOT                        T      NS      %        =                            T          TOT                -                              ∑                          i              =              1                                      N              activecells                                ⁢                                    ∑                              j                =                1                            S                        ⁢                          T              j                              (                i                )                                                                T        TOT            wherein:
Nactivecells is the number of cells being present in the solution;
S is the number of services being taken into account;
ATOT is the area covered by the complete set of candidate cells;
TTOT is the traffic carried by the complete set of candidate cells;
Aiservice is the area covered by the i-th activated cell; and
Tj(i) is the traffic, of the j-th type, carried by the i-th activated cell, and                the further cost items SC and SSH represent an indication of the mean square deviation (or standard deviation) of cell loads ηicell and soft hand-over loads ηiSHO of the activated cells, by ideal load and soft hand-over loads.        
The shown function does not guarantee the definition of a final solution that globally optimises the network planning in the considered geographic area.
The Applicant deems that the global planning optimisation is fundamentally important for providing the services according to minimum capillarity requirements of network on territory, such fact not being guaranteed by the prior art either due to coverage limits or due to traffic limits.
According to the Applicant, the current prior art therefore has a series of particularly relevant problems related to the impossibility of jointly and efficiently taking into account the design constraints, such as, for example, the minimum geographic area in which the provided services (namely a predetermined volume of traffic) and the minimum traffic to be carried for the set of considered services are guaranteed.
Moreover, the Applicant notes that the prior art as a whole does not take into account a particular constraint about the characteristics of the optimum defined solution, namely the predetermination of compulsorily active cells (namely the cells not subjected to the selection process).
The impossibility of deeming as already active, and therefore locked, particular cells, makes the known methods, and in particular those mentioned as most relevant, unable for preparing the network “roll-out”, within which it is mandatory to take into account the set of cells already active in the previous years, in order to develop (enlarge) the network in time.
In the specific context of third-generation mobile networks, a further limit of the mentioned known prior art is given by the definition of a cost function to be minimised, used in the optimisation procedure that does not allow any evaluation of the possible pilot pollution phenomenon (interference from pilot channel).
Like every cellular radio-mobile system, the third-generation UMTS system provides, as known, for common control channels that are broadcast within the whole area of each cell. Such channels transmit mandatory system information for receiver terminals.
Among these, the Common Pilot Channel (CPICH) is a physical channel in down-link (from radio station to terminals) that is used by mobile terminals for network synchronisation. The pilot channel signals operates as “beacon” to point out the existence of a base station to network receiver terminals.
Each cell transmits its own pilot signal at a common frequency; the comparison between signal powers from different pilot channels allows the terminals to recognise the server base station and to manage possible hand-over processes. It is provided that, in particular areas, called macro-diversity areas, a mobile terminal decodes the signal from many antennas, and therefore to exchange information with many Base Radio Stations (SRB).
If a terminal is found in an area in which it receives the pilot signals from a number of cells greater than the maximum number of cells that can be managed by the terminal and with comparable powers, an interference phenomenon results, known as “pilot pollution”, that can generate a decrease transmission capability in the area or also the loss of current calls. The phenomenon also implies a higher consumption of internal terminal power due to the increase of processing needs. In fact, in such situation, the mobile terminal continuously changes the set of cells to which it is connected in macro-diversity (the so-called “active set”), because the number of candidate cells to macro-diversity is greater than the maximum number of cells that can be managed by it.
The pilot pollution is therefore an indicator of how many cells in excess are perceived by a radio-mobile terminal when it is connected to a network with respect to its cell managing capability (maximum number of cells to which it can be connected in macro-diversity) and with respect to network parameters.
The evaluation of the pilot pollution phenomenon, related to the system down-link section, is fundamentally important for a good design/planning of a third-generation network, and therefore it must be taken into account in a design/planning context to allow the correct and efficient operation of mobile terminals in the network as designed/planned.